Permanent-magnet field synchronous motor

ABSTRACT

A permanent-magnet field synchronous motor comprises a rotor body (22) having a cylindrical outer surface. A plurality of permanent magnets (25) attached to the cylindrical outer surface of the rotor body and arranged in the circumferential direction thereof form a field system. Each of the permanent magnets has an outer surface (25b) which extends circumferentially in parallel to the cylindrical outer surface of the rotor body and also has a constant thickness in the whole thereof. The inner surface and the outer surface of each of the permanent magnets has a certain configuration, respectively, each of which is composed of a plurality of straight lines and approximates to a contour line consisting of half-sine curves (S 1 , S 2 ) extending between the circumferential opposite ends of each of the permanent magnets when developed in a plane surface.

TECHNICAL FIELD

The present invention relates to an improvement of a permanent-magnetfield synchronous motor comprising a rotor body having a cylindricalouter surface and a plurality of permanent magnets as a field systemattached on the outer surface of the rotor body and arranged in acircumferential direction thereof.

BACKGROUND ART

A permanent-magnet field synchronous motor comprising a rotor bodyhaving a cylindrical outer surface and a plurality of permanent magnetsas a field system attached on the outer surface of the rotor body andarranged in a circumferential direction thereof has been known. In thiskind of motor, each of the permanent magnets is magnetized so thatmagnetic poles are formed on the inside and outside surfaces thereof,respectively, and that the magnetic poles on the outside surfaces of thepermanent magnets are alternately different in the circumferentialdirection.

When using such a permanent-magnet field synchronous motor as a servomotor, it is necessary to reduce fluctuation of a torque produced in themotor as much as possible in order to smoothly rotate a rotor assembly.Therefore, it is desirable that a characteristic of electromotive forceproduced in the windings of the stator changes sinusoidally in relationto a time. For this, as shown in FIG. 1, a construction in which a gapdefined between the outside surfaces of the permanent magnets 12 fixedto the cylindrical outer surface of the rotor body 11 and thecylindrical inner surface of the stator 13 changes sinusoidally inaccordance with rotation of the rotor body 11, has been adopted for amotor.

In the motor having such a construction, however, since the innersurface of the stator 13 is substantially a cylindrical surface, it hasbeen necessary to give the outside surface of each of the permanentmagnets 12 a specially curved shape in order to obtain such a gapsinusoidally changeable between the cylindrical inner surface of thestator and the permanent magnets. Generally, the permanent magnet hasbeen made by a press sintering. When making a permanent magnet havingsuch a specially curved shape, it is necessary to make a press moldhaving a specially curved concavity. Therefore, processing of such apermanent magnet is difficult. Further, a grinding process of theoutside surface of such a permanent magnet, performed after moldingthereof, is also difficult. Therefore, the processing accuracy of thepermanent magnet is decreased. Furthermore, since such a permanentmagnet has a thickness which becomes gradually thinner toward theopposite ends thereof in a circumferential direction, a pressing forcetends to be applied unevenly to the permanent magnet, and thus aninternal stress in the permanent magnet is not uniformly distributed.Consequently, the motor has a disadvantage in that the permanent magnet,being hard and brittle in general, is apt to be easily broken.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a permanent-magnetfield synchronous motor in which a characteristic of electromotive forceproduced in the windings of the stator changes sinusoidally in relationto time, by the use of permanent magnets having a constant thickness.

According to the present invention, there is provided a permanent-magnetfield synchronous motor comprising: a rotor body having a cylindricalouter surface; and a plurality of permanent magnets attached to saidcylindrical outer surface of said rotor body, each of said permanentmagnets having an outer surface which extends circumferentially inparallel to said cylindrical outer surface of said rotor body and alsohaving a constant thickness in the whole thereof, said outer surface andan inner surface of each of said permanent magnets having a certainconfiguration, respectively, each of which is composed of a plurality ofstraight lines and approximates to a contour line consisting ofhalf-sine curves extending between the circumferential opposite ends ofeach of said permanent magnets when developed in a plane surface.

According to the construction of the present invention, although theouter surface of each of the permanent magnets and the inner cylindricalsurface of a stator define therebetween a constant gap in thecircumferential direction thereof, the width of each of the permanentmagnets in the axial direction thereof increases and then decreases inthe circumferential direction along the contour lines approximate tohalf-sine curves illustrated in a cylindrical surface. Consequently, acharacteristic of a magnetic flux density passing through the windingsof the stator changes sinusoidally in relation to time while thepermanent magnets as a field system rotate together with the rotor body.As a result, a fluctuation of torque produced by the motor is decreasedand thus a smooth rotation-output is obtained. The outer surface of eachof the permanent magnets extends in parallel to the cylindrical outersurface of the rotor body to define a constant thickness thereof, andthe configuration of each of the permanent magnets is composed of aplurality of straight lines. Therefore, a press mold for the permanentmagnets can be easily made, and the outer surface of the permanentmagnet can be easily processed with a high processing accuracy. Further,since each of the permanent magnets has a constant thickness in thewhole thereof, a pressing force can be evenly applied to the permanentmagnet. Therefore, the break of the permanent magnet can be prevented,and anisotropy in the permanent magnets can be increased. As a result,the magnetic property of the permanent magnets can be improved, and thusthe magnetic property of the permanent-magnet field synchronous motorcan be improved.

Preferably, the inner surface and outer surface of each of the permanentmagnets have a certain configuration, respectively, each of whichconfigurations is composed of straight lines and is inscribed in orcircumscribed on a virtual contour line consist of a pair of half-sinecurves extending symmetrically along the circumferential center line ofeach of the permanent magnets. More preferably, the inner surface andouter surface of each of the permanent magnets have symmetricalconfigurations, respectively, in relation to the circumferential andaxial center lines thereof. When using such a configuration, eachpermanent magnet has a configuration which is substantially oval inshape, and thus a pressing force applied to the permanent magnet can bemore uniformly distributed during the press sintering thereof.

Preferably, a plurality of permanent magnets having the sameconfiguration are spaced from each other in the circumferentialdirection of the rotor body and arranged adjacent to each other in thedirection of rotation axis of the rotor body. When using such anarrangement, the permanent magnets can be arranged on the cylindricalouter surface of the rotor body with a high density. Therefore, amagnetic flux density in the whole of the field system can be increased.

The foregoing and other features and advantages of the present inventionwill become apparent from the following detailed description withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematical cross-section view illustrating a constructionof the conventional permanent-magnet field synchronous motor;

FIG. 2 is a schematical cross-section view of the permanent-magnet fieldsynchronous motor illustrating an embodiment of the present invention;and

FIG. 3 is a development view of the configuration of the permanentmagnet attached on the outer surface of the rotor body of the motorshown in FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIGS. 2 and 3, the permanent-magnet field synchronous motorcomprises a stator 21 having therein a cylindrical bore, and a rotorbody 22 arranged rotatably in the bore of the stator 21. The rotor body22 has a cylindrical outer surface which is arranged coaxially with thecylindrical inner surface of the stator 21. The cylindrical innersurface of the stator 21 is formed with slots 23 extending in the axialdirection. Stator-windings 24 are passed through the slots 23. Aplurality of permanent magnets 25 forming a field system are fixed tothe cylindrical outer surface of the rotor body 21.

Each permanent magnet 25 has an inner surface 25a extendingcircumferentially so as to fit the cylindrical outer surface of therotor body 22, and has a constant thickness in the whole thereof so thatan outer surface 25b of each of the permanent magnets 25 and thecylindrical inner surface of the stator 21 define therebetween aconstant gap. That is, the outer surface 25b of each of the permanentmagnets 25 extends in parallel to the outer surface of the rotor body22.

As shown in FIG. 3, the outer surface 25b of each of the permanentmagnets 25 has a configuration composed of a plurality of straight linesso that it approximates to an imaginary contour line consist of a pairof half-sine curves S₁ and S₂ when developed in a plane surface. Theinner surface 25a of each of the permanent magnets 25 has the sameconfiguration as that of the outer surface 25b thereof. Further, theconfiguration of the inner and outer surfaces 25a and 25b of each of thepermanent magnets 25 consists of a pair of contour line sections whichare symmetrical to each other in relation to a longitudinal center lineC₁ parallel to the rotation axis of the rotor body 22, and to acircumferential center line C₂ perpendicular to the longitudinal centerline C₁.

In more detail, the configuration of the inner and outer surfaces 25aand 25b of each of the permanent magnets 25 forms, in this embodiment,and octagonal shape which inscribes the above-mentioned half-sine curvesS₁ and S₂ when developed in a plane surface.

The permanent magnets 25 having the above-described configuration arearranged and spaced from each other in the circumferential direction ofthe rotor body 22. The permanent magnets 25 are also arranged adjacentto each other in the axial direction of the rotor body 22.

In the permanent-magnet field synchronous motor having theabove-described construction, the configuration of the inner and outersurfaces 25a and 25b of each of the permanent magnets 25 is composed ofa plurality of straight lines so that it approximates to an imaginarycontour line consist of a pair of half-sine curves S₁ and S₂ whendeveloped in a plane surface. Consequently, the density of the fieldflux passing through the stator-windings 23 is changed sinusoidallyduring the rotation of the permanent magnets 25 on the rotor body 22,although the outer surface of each of the permanent magnets 25 and thecylindrical inner surface of the stator 21 define therebetween aconstant gap in the whole thereof. Therefore, the fluctuation of thetorque produced by the motor is decreased, and thus a smooth-rationaloutput of the motor is obtained.

Since the inner and outer surfaces 25a and 25b of each of the permanentmagnets 25 are partially cylindrical surfaces, respectively, a pressingmold for the permanent magnets can be easily manufactured. Further,since the outer surface 25b of each of the permanent magnets 25 is apartially cylindrical surface, a finish-grinding process can be easilyapplied to the outer surface after molding thereof, and thus a highprocessing accuracy can be obtained.

Further, since the configuration of the inner and outer surfaces 25a and25b of each of the permanent magnets 25 is not composed of a pair ofhalf-sine curves S₁ and S₂, but composed of a plurality of straightlines so that it approximates thereto when developed in a plane surface,the configuration of the permanent magnets 25 can be easily processed.

Furthermore, since each permanent magnet 25 has a constant thickness inthe whole thereof, unevenness in the pressing force applied thereon doesnot easily occur during the press sintering thereof, and thus thepermanent magnet 25 is not easily broken.

The process of the permanent magnet 25 is accomplished in such a mannerthat the inner and outer surfaces 25a and 25b of the permanent magnet 25are magnetized to form thereon an N or S pole after the press sinteringand the subsequent machining of the permanent magnet 25 is finished.Since the pressing force is uniformly added to the permanent magnet 25in the thickness direction thereof, and thus the internal stress isuniformly distributed in the permanent magnet 25 during the presssintering thereof, a permanent magnet 25 having a high anisotropy can beobtained.

As a result, the strength and the magnetic characteristics of thepermanent magnet 25 can be increased. Therefore, permanent magnets 25,which are expensive in general, can be made thin and thus the costthereof can be reduced.

Although the foregoing explanation has been made in reference to oneembodiment of the present invention, the present invention is notlimited to the above-mentioned embodiment. For example, theconfiguration of the inner and outer surfaces of each of the permanentmagnets is not limited to and octagonal shape when developed in a planesurface, and it may be any other polygonal shape, it also may be anypolygonal shape circumscribed with half-sine curves. Further, thepermanent magnet may have a configuration such as obtained when thepermanent magnet shown in FIG. 3 is divided into two pieces along thecircumferential center line thereof. Furthermore, although the abovementioned embodiment includes a field system having four poles, thepresent invention is not limited thereto.

CAPABILITY OF EXPLOITATION IN INDUSTRY

As apparent from the foregoing explanation, the permanent magnetsaccording to the present invention can be easily manufactured and thestrength and the magnetic properties of the permanent magnets can beincreased. Therefore, the permanent-magnet field synchronous motorhaving excellent magnetic properties can be cheaply provided.

We claim:
 1. A permanent-magnet field synchronous motor comprising:arotor body having a cylindrical outer surface; and a plurality ofpermanent magnets attached to said cylindrical outer surface of saidrotor body, each of said permanent magnets having a constant thickness,such that an outer surface thereof extends circumferentially parallelwith said cylindrical outer surface of said rotor body, each permanentmagnet having a polygonal shape, said polygonal shape having a shapeapproximating that formed by a contour line superimposed upon said rotorbody, said contour line having the shape of a half-sine curve above acenter line superimposed circumferentially around said rotor body andsaid contour line having a mirror image of the half-sine curve below thecenterline, said polygonal shape also being inscribed in said contourline, and wherein each of said plurality of permanent magnets have thesame configuration and are arranged and spaced from one anothercircumferentially around said rotor body, and are arranged with at leastone of said permanent magnets being adjacent to another in the axialdirection of the rotor body.
 2. The permanent-magnet field synchronousmotor of claim 1, wherein each of said permanent magnets is symmetricalwith respect to a circumferential center line and to an axial centerline of said permanent magnet.